1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus. More specifically, the present invention relates to a configuration for feeding sheets, such as envelopes, having a non-uniform thickness.
2. Description of the Related Art
At present, an image forming apparatus, such as a copying machine, printer, and facsimile, in which an image forming portion forms an image on a sheet fed from a sheet feeding apparatus, has been widely used. In the sheet feeding apparatus, typically, a sheet feeding cassette which accommodates sheets therein is detachably attached to an apparatus body so that a pickup roller provided in the apparatus body automatically feeds the sheets accommodated in the sheet feeding cassette.
In order for the pickup roller to press and feed the sheets stacked on the tray of the sheet feeding cassette, the sheet feeding apparatus has a lifting and lowering portion which can lift and lower the tray which stacks the sheets thereon. Further, to accommodate different-sized sheets, the sheet feeding cassette has a slidable trailing end regulating portion which regulates the position of an upstream end in a sheet feeding direction of the sheets stacked on the tray (hereinafter, a trailing end). The sheet feeding cassette also has a pair of side end regulating portions which regulate side end positions in a direction perpendicular to the sheet feeding direction of the sheets stacked on the tray (hereinafter, a width direction).
The pair of side end regulating portions regulates the side ends of each sheet on the tray, and the trailing end regulating portion regulates the trailing end of each sheet on the tray, thereby positioning the position of each sheet into a predetermined position. Each sheet is thus positioned on the tray of the sheet feeding cassette. The tray is then lifted by the lifting and lowering portion to press the pickup roller onto the stacked sheets, so that the pickup roller is rotated to feed the sheets.
Conventionally, to continuously feed envelopes in the sheet feeding apparatus, an envelope-specific unit is attached to the sheet feeding cassette to continuously feed the envelopes accommodated in the envelope-specific unit by the pickup roller. The envelope-specific unit urges an auxiliary tray which stacks the envelopes thereon upward by a spring to press the envelopes on the auxiliary tray onto the pickup roller by the resilient force of the spring (see Japanese Patent Laid-Open No. 10-203658). The pickup roller is then rotated to feed the envelopes.
Although, conventionally, the envelopes are fed using the envelope-specific unit, it is considered, from a request to simplify the envelope feeding configuration, that the envelopes are fed using the lifting and lowering portion of the tray of the sheet feeding cassette without using the spring.
However, as illustrated in FIG. 9A, envelope PA has a flap 14 which closes its inlet. The flap 14 covers part of envelope PA, not the entire surface of envelope PA, so that the portion of envelope PA which is covered by the flap 14 is thicker than the portion of envelope PA which is not covered by the flap 14. That is, the sheet is folded on its front and back sides to be formed in a bag shape, thereby folding the flap 14 on its end, so that envelope PA has the portion having a thickness of two sheets and the portion having a thickness of three sheets.
Therefore, when envelopes PA are stacked in the same direction, as illustrated in FIG. 9B, the bundle of envelopes is thick on the flaps 14 side to have a non-uniform thickness. Suppose that the tray of the sheet feeding cassette is lifted to press the bundle of stacked envelopes onto the pickup roller. As illustrated in FIG. 10, a pickup roller 104 is pressed onto one side in the width direction of the upper surface of the bundle of envelopes PA, so that the feeding pressure becomes non-uniform. As a result, sheet feeding failure due to insufficient feeding pressure and conveying failure such as sheet skew feeding and jamming are caused.
Accordingly, the present invention has been made in view of the circumstances, and it is desirable to provide a sheet feeding apparatus and an image forming apparatus which can reliably feed even sheets, such as envelopes, having a non-uniform thickness.